In a currently popular vision of the future development of communication in cellular networks, huge numbers of (mostly) small autonomous devices become increasingly important. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers (which configure the devices and receive data from them) within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication and the devices may be denoted machine devices. In the 3GPP standardization, the corresponding alternative terms are machine type communication (MTC) and machine type communication devices (MTC devices), with the latter being a subset of the more general term user equipment, UE. In terms of numbers MTC devices will dominate over human users, but since many of them will communicate very scarcely, their part of the traffic volume will be much smaller than their part of the “user” population.
With the nature of MTC devices and their assumed typical uses follow that they will often have to be very energy efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries. For such energy deprived devices, the traffic is characterized by small, more or less infrequent transactions (often delay tolerant), which will result in a large signaling overhead. Hence, reducing the signaling overhead is an important means to facilitate for such devices to efficiently function, with a long battery lifetime, using a cellular network.
Machine devices, however, consist of a very heterogeneous flora of devices and applications. Although the above described energy deprived (e.g. sensor) devices may, according to the vision, constitute the largest part in terms of numbers, many other types of MTC devices and MTC applications are also envisioned or already existing. One area that has received quite a lot of attention is the development of power grids into what is denoted as “smart grids”. This refers to the evolution of the conservative power grid technology into grids that are better adapted to the envisioned future requirements in the area of generation and distribution of electricity, involving intermittent generation sources such as wind and solar power plants, many small generation sources (such as customers which sometimes produce more electricity than they consume) and a desire to impact the customers energy consumption habits to even out load peaks. In this evolution information technology, in particular communication technology has an important role to play. In many smart grid applications entities in the power grid, so-called substations (e.g. transformer stations) communicate with each other and with a control center for the purpose of automation and protection of equipment when faults occur. In contrast to the above described energy deprived devices with delay tolerant scarce communication, these smart grid applications often have extremely strict latency requirements, the amount of data communicated may range between small and large and the energy supply is typically not a major issue. To make cellular communication technology a possible and attractive means of communication for such devices and applications, it is crucial to keep the delay associated with access and end-to-end communication as low as possible.
As the present disclosure is related to scheduling/allocation of mainly uplink transmission resources for wireless devices or mobile terminals (denoted user equipments, UEs in 3GPP systems), a brief description of how scheduling/allocation of uplink transmission resources is performed in LTE follows below.
The procedure leading to an uplink transmission of data on the Physical Uplink Shared Channel (PUSCH) comprises a request from the UE for uplink transmission resources, an allocation of uplink resources signaled from the eNB triggered by the request, and finally an uplink transmission of data from the UE. This procedure is illustrated in FIG. 2.
As illustrated in FIG. 2, the resource request from the UE is typically in the form of a scheduling request (SR) transmitted on Physical Uplink Control Channel (PUCCH) resources dedicated for the UE. The SR in itself contains no structure and no specific information other than that uplink transmission resources are requested. The PUCCH resources that are used for the SR transmission implicitly indicate which UE is requesting the transmission resources, since these PUCCH resources are dedicated for this UE. The scheduling algorithm in the eNB then selects suitable uplink transmission resources on the PUSCH to allocate to the UE, and signals the allocation to the UE using an uplink grant on the Physical Downlink Control channel (PDCCH). The uplink grant is addressed to the concerned UE by adding the dedicated Cell Radio Network Temporary Identifier (C-RNTI) of the UE to the Cyclic Redundancy Check (CRC) of the Downlink Control Information (DCI) containing the uplink grant. In other words, the C-RNTI is not explicitly included in the DCI. In the current release of LTE, the allocation always concerns resource blocks (which are indicated in the uplink grant) occurring four subframes later than the uplink grant. However, other releases or other systems may of course apply a different time interval between the uplink grant and the allocated resources. The last step is that the UE transmits buffered uplink data, i.e. uplink data waiting for transmission, using the allocated uplink resources on the PUSCH.
In addition to the above described regular one-time allocation of transmission resources there is a special form of allocation of repetitive transmission resources denoted semi-persistent scheduling (SPS). SPS may be configured in advance for a UE through RRC signaling. In practice, configuration may be performed through the SPS-Config IE in the radioResourceConfigDedicated IE using the RRCConnectionSetup message or an RRCConnectionReconfiguration message. The SPS configuration mainly consists of allocation of a UE-specific SPS-C-RNTI and a repetition interval (for either or both of uplink and downlink as applicable) for the resources to be allocated through SPS. The actual allocation of the repetitive SPS resources is communicated in an uplink grant, with the contents of a one-time allocation, but addressed to the SPS-C-RNTI of the concerned UE. The repetitive resources allocated through SPS may be explicitly released through PDCCH signaling in the form of another uplink grant addressed to the same SPS-C-RNTI with dummy parameter values to indicate ‘SPS release’. This is typically done if a Buffer Status Report (BSR) indicating an empty buffer is sent for a configurable number (e.g. 2 or 3) of consecutive uplink transmissions.
Another way to request PUSCH transmission resources is that the UE transmits a Buffer Status Report (BSR) indicating a non-empty transmission buffer. A BSR is conveyed in the form of a MAC Control Element, typically transferred to the eNB in conjunction with a user data transmission, wherein the MAC Control Element containing the BSR is a part of the MAC PDU that carries the user data.
In addition to the above described procedures for request and allocation of transmission resources, uplink transmission resources may be allocated using the Random Access Response message during the random access procedure, thus making a random access preamble transmission effectively work as a request for transmission resources.
Uplink transmission resources, whether allocated via SPS or using regular one-time allocation, are allocated as one or more chunks of the OFDM time-frequency grid, denoted resource blocks. A resource block consists of 12 subcarriers of 15 kHz each in the frequency domain and a slot of length 0.5 ms in the time domain. Pairs of slots are further grouped together to form 1 ms subframes. Each slot consists of 7 resource elements (or 6 if an extended cyclic prefix is used), each containing an OFDM symbol including cyclic prefix. Hence, each resource block consists of 84 resource elements (or 72 if the extended cyclic prefix is used). Scheduling in LTE is performed on a subframe basis, i.e. in each subframe the available bandwidth may be allocated to one or more UEs. In the frequency domain the allocations have to adhere to resource block boundaries, as mentioned above. The smallest possible allocation is thus two resource blocks, one in each slot of a subframe. When data is transmitted using the allocated resources, the modulation and coding scheme and transport format are chosen such that they match the size of the allocated resources, and the bits to be transmitted are mapped to the resource elements (and OFDM symbols) of the allocated resource. In order to enable coherent detection at the receiver (e.g. an eNodeB), a transmitting UE includes a cell specific Demodulation Reference Signal (DMRS) time-interleaved with the data. Specifically, the DMRS is transmitted in the fourth (or third if an extended cyclic prefix is used) OFDM symbol of each slot, i.e. twice every subframe, across the entire allocated transmission resource, i.e. on all subcarriers of the allocated resource blocks. From 3GPP release 11, a DMRS may be made UE specific based on a combination of DMRS sequence and phase rotation of the sequence.
US 2012/0275421 A1 discloses a technique for reducing a time delay between an application output at a subscriber station and uplink resource allocation for the subscriber station. The technique includes scheduling one or more probe uplink resource allocations for the subscriber station, and scheduling subsequent periodic uplink resource allocations based on at least one of the probe uplink resource allocations.
There is still a need in the art for improved mechanisms for allocating uplink transmission resources to wireless devices that are transmitting periodic uplink data traffic.